1. Field of the Invention
The present invention relates to an optical material used for optical elements such as a lens, a filter, a mirror, and a refraction optical element, and more particularly to an optical material with large refractive index dispersion. Further, the present invention relates to an optical element formed by the optical material and a method of forming the optical element.
2. Related Background Art
Conventionally, an optical system comprising a plurality of lenses for refracting a light beam decreases chromatic aberration by combining glass materials with different dispersion characteristics. Objective lenses of a telescope, for example, comprise a positive lens using a small-dispersion glass material and a negative lens using a large-dispersion glass material. These lenses are combined to correct chromatic aberration appearing on an axis. When the lens configuration or the number of lenses is restricted or glass materials to be used are limited, there has been a case where chromatic aberration cannot fully be corrected.
In contrast to the method of decreasing chromatic aberration by combining glass materials as mentioned above, there is disclosed a method of decreasing chromatic aberration by providing a diffraction grating to part of a lens or optical system. For example, the latter method is disclosed in SPIE Vol. 1354 International Lens Design Conference (1990) and the like. In an optical system, it is known that the refraction surface and the diffraction surface cause chromatic aberrations reverse to each other against a light beam having a given reference wavelength. This phenomenon is used to offset both chromatic aberrations and decrease the entire chromatic aberration. Such diffraction optical element can provide the function equivalent to an aspheric lens by continuously changing the cycle of the diffraction grating structure. The use of the lens having such diffraction grating can greatly decrease chromatic aberration.
On a normal refraction surface, one light beam is unchanged after refraction. By contrast, one light beam is diffracted on a diffraction surface and is separated into a plurality of light beams corresponding to the number of orders. When the diffraction optical element is used as an optical system, the grating structure needs to be determined so that the diffracted light beams allow a luminous flux in an available wavelength area to concentrate on a designed specific order. When the light beam concentrates on the specific order, diffraction light intensities are low for the other orders. When the intensity is 0, the diffraction light is not present. On the contrary, when there is a light beam having an order other than the specific order, that light beam forms an image at a position different from the light beam corresponding to the specific order, generating flare. In order to efficiently decrease chromatic aberrations, it is necessary to fully increase the diffraction efficiency of the light beam corresponding to the specific order.
Japanese Patent Application Laid-Open Nos. 9-127321, 9-127322, 11-044808, 11-044810, and the like disclose configurations to determine the grating structure so that the luminous flux in an available wavelength area concentrates on a specific order. These configurations select a plurality of materials having different dispersions and an optimum thickness of each diffraction grating to provide high diffraction efficiency in a wide wavelength range. More specifically, a plurality of different optical materials are laminated on a substrate. On at least one of interfaces therebetween, a relief pattern, a step shape, a kinoform, and the like are formed to configure an intended multilayer diffraction optical element.
According to Japanese Patent Application Laid-Open Nos. 9-127321, 9-127322, 11-044808, 11-044810, however, there is used a combination of materials having relatively low and high refractive index dispersions in order to obtain the configuration having high diffraction efficiency in a wide wavelength range.
More specifically, Japanese Patent Application Laid-Open No. 9-127321 uses a combination of BMS81 (nd=1.64, νd=60.1: manufactured by OHARA) and plastic optical material PC (nd=1.58, νd=30.5: manufactured by TEIJIN CHEMICALS). Japanese Patent Application Laid-Open No. 9-127322 uses a combination of LaL14 (nd=1.6968, νd=55.5: manufactured by OHARA), acrylic resin (nd=1.49, νd=57.7), Cytop (nd=1.34149, νd=93.8: manufactured by ASAHI GLASS CO., LTD.), and PC (nd=1.58, νd=30.5: manufactured by TEIJIN CHEMICALS). Japanese Patent Application Laid-Open Nos. 11-044808 and 11-044810 use a combination of C001 (nd=1.5250, νd=50.8: manufactured by Dainippon Ink & Chemicals, Inc.), plastic optical material PC (nd=1.58, νd=30.5: manufactured by TEIJIN CHEMICALS), PS (nd=1.5918, νd=31.1), PMMA (nd=1.4917, νd=57.4), and BMS81 (nd=1.64, νd=60.1: manufactured by OHARA).
If an optical element such as the diffraction optical element, due to its shape, causes the light to form a large incidence angle (field angle), the light is shaded to generate flare or ghost. In order to increase the field angle, it is necessary to use a material having a refractive index dispersion greater than that for conventional optical materials. FIG. 9 is a graph showing Abbe numbers and refractive indexes of optical materials that are commercially available or under research and development. The ordinate axis indicates a refractive index (nd). The abscissa axis indicates an Abbe number (νd). Of these materials, it is known that polyvinylcarbazole, one of organic polymers, indicates the smallest Abbe number 17.3.
Japanese Patent Application Laid-Open No. 10-268116 describes an example of using polyvinylcarbazole for the multilayer diffraction optical element. In Japanese Patent Application Laid-Open No. 10-268116 lists polyvinylcarbazole as an elastic resin, as seen from table 3. Polyvinylcarbazole, an elastic resin, is pressed against the diffraction grating and is bonded to the surface of the diffraction grating through pressure to be united with it, thus forming a multilayer diffraction grating. Since polyvinylcarbazole is a very fragile material, a small load can easily crack it, for example. Accordingly, polyvinylcarbazole as an elastic resin needs to be pressed to the diffraction grating for a considerably long time in order to form an optical element as described in Japanese Patent Application Laid-Open No. 10-268116. This is industrially very difficult. In recent years, diffraction gratings become finely and complexly shaped, making such method much more difficult.
On the other hand, polyvinylcarbazole can be formed by applying a polymerization reaction to N-vinylcarbazole (νd=17.8). Since N-vinylcarbazole is fluid, the raw material can be easily filled into even gaps of a mold having a complicated diffraction grating structure. Accordingly, it is considered an effective method to use N-vinylcarbazole as an optical material for optical elements having complicated shapes such as a diffraction grating.
Although diffraction optical elements and the like can be molded into any shapes by applying light polymerization, N-vinylcarbazole is crystallized at room temperature and cannot directly be used for replica molding that is usually performed at room temperature.
As one of conventional optical element molding techniques, the replica molding is appropriate for production of large-area moldings and excels in transferability. Due to its ease of molding, the replica molding is suitable for mass production. During the replica molding, photo-curing resin is dropped on a molding surface that is shaped reversely to an intended optical shape. A lens blank is pressed onto the photo-curing resin to spread it. When the photo-curing resin is shaped as intended, light from a light source is applied to cure the photo-curing resin. The cured photo-curing resin is separated from the mold to complete the molding. Generally, however, the replica molding is conducted at room temperature. If the replica molding uses an optical material such as N-vinylcarbazole that crystallizes at room temperature, the crystallization starts on dropping the N-vinylcarbazole into the mold under room temperature and advances while the lens blank is pressed. The optical element cannot be shaped as intended.
A possible solution for the replica molding is to increase a molding temperature higher than the crystallization temperature for suppressing crystallization of the optical material that crystallizes at room temperature (normally 22° to 28° C.). However, molds for the replica molding are generally restricted. Accordingly, the mold and the optical element, when heated, cannot expand freely, causing nonlinear expansion. It is very difficult to control shapes. Controlling shapes becomes more difficult when the mold has temperature distribution. In addition, recent optical element shapes are so complicated as to have free curved surfaces or grating forms. Therefore, it is difficult to manufacture a mold in consideration of thermal expansion of the mold and the optical element.
There is another problem about N-vinylcarbazole for its low viscosity. During replica molding, the photo-curing resin is dropped. Conventionally, this process uses a metered-dose discharger called a dispenser. The dispenser drops the photo-curing resin at a constant amount to control the thickness or area of a molded surface. If the resin is dropped at an irregular amount, it is impossible to obtain an intended area or thickness. The resin flows over to the mold side to generate a mold flash. The viscosity of N-vinylcarbazole is low, i.e., 4 mPa·s or lower at the melting point (approximately 65° C.) or higher. It is difficult to control the amount of drops. In order to control the viscosity, adding other compounds such as a thickener adversely affects the refractive index dispersion, degrading the performance.